1. Field of the Invention
The invention relates to measuring apparatus, and more particularly to length measuring apparatus.
2. General Background
The External Tank 2 (FIGS. 1 and 2) of the Space Shuttle System 1 is a very large cryogenic tank with a relatively thin wall. Tank 2 is generally made of welded aluminum (2219) and is nominally 331.000 inches in diameter (86.656 feet or 1,039.867 inches in circumference). The wall of tank 2 is nominally 0.200 inches thick (1/8% of the thickness of its diameter). If tank 2 were reduced proportionally so that its diameter were equal to that of a soft drink can, the tank 2 wall would be one half the thickness of the soft drink can wall. External Tank 2 comprises a number of cylindrical barrel sections 4 and dome structures 6 which are welded together. Because of the thinness of the wall, the perimeters of two barrel sections 4 which are to be welded together must be manufactured within close tolerance (.+-.0.02 percent). If not, the out-of-tolerance section 4 may have to be discarded, which is a very expensive proposition.
The current method of perimeter measurement of tanks such as tank 2 involves using a steel tape. The steel tape is pulled with constant tension around the surface to be measured. Care is taken to ensure that the tape remains in the same measurement plane. The temperature is determined using a hand-held surface probe. Several measurements of the surface temperature are taken and averaged to determine the nominal temperature. The measurement is then corrected to account for the difference between the nominal temperature and some reference temperature. It normally takes approximately one hour to perform the steps necessary to obtain one measurement. Anomalies in the steel tape used for the measurement and defects in the surface to be measured can adversely affect the measurement. Since the measurement is technique-dependent, it is difficult to maintain a high degree of repeatability without significant training. Thus, using a steel tape, it is difficult to obtain a repeatable and accurate measurement of the perimeter of an External Tank section 4.